Speed control effect doesn't change the length of video strip
When you add a Speed Control strip, Stretch to input strip length is enabled by default.  That gives you the ability to change the length of your video strip (aka "input strip") and the speed will automatically adjust.  For example, if you make your video strip half as long, the speed will be twice as fast.

You also can set the parameter Multiply Speed.  This will change the speed of your video without changing the length.  For example, you can set this to 2.0, but since the video finishes twice as fast yet the length stays the same, the video will run out before the end of the strip, and so for the remainder of the strip, Blender will just display the last frame.